1. Field of the Invention
The present invention relates to a color linear image sensor device with a shutter function for charges generated by a photodetector, and a method of manufacturing such a color linear image sensor device.
2. Description of the Related Art
Color linear image sensors are semiconductor devices having a plurality of parallel CCD linear image sensors disposed on a semiconductor substrate and having a charge transfer function. CCD linear image sensors comprise arrays of photodetectors with color filters of different colors (e.g., three color filters of the colors GREEN, BLUE, RED) mounted thereon.
Color linear image sensors have functions of converting incident light into charges and of successively outputting the charges, and are being widely used as a vital device in color scanners and color copying machines.
In actual color scanners and color copying machines, the color linear image sensor is mechanically scanned in a direction (feed direction) perpendicular to the direction (main scanning direction) of the arrays of photodetectors of the color linear image sensor for generating color information of an image in given positions on a subject. Color information of the image in given positions on the subject is produced for each of the colors using line outputs 1, 2, 3, . . . .
FIG. 1 is a view showing an overall arrangement of a conventional color linear image sensor. The color linear image sensor shown in FIG. 1 has no charge control function (shutter function).
As shown in FIG. 1, the conventional color linear image sensor comprises photodetector circuits 1001a-1001c have R, G, B color filters (not shown) mounted thereon for photoelectrically converting received light into charges, charge read circuits 1002a-1002c for reading the charges generated and stored by the photodetector circuits 1001a-1001c, and charge transfer circuits 1003a-1003c for transferring the charges read by the charge read circuits 1002a-1002c. 
The charge transfer circuits 1003a-1003c usually comprise CCD shift registers that are driven by two-phase drive clocks xc3x81, xc3x82. The two-phase drive clocks xc3x81, xc3x82 are supplied from pulse lines L1001a-L1001c, L1002a L1002c that are disposed closely to the charge transfer circuits 1003a-1003c. The charges transferred by the charge transfer circuits 1003a-1003c, which are actually formed by floating diffused regions, are outputted from output circuits 1004a-1004c each comprising a charge detector for converting charges into voltages and an analog circuit including a source follower, an inverter, etc.
FIG. 2 is a view showing a detailed structure of the conventional color linear image sensor shown in FIG. 1. In FIG. 2, a lower figure section is an enlarged view of an area X4 indicated by the dotted lines in an upper figure section.
In FIG. 2, the color linear image sensor comprises polycrystalline silicon electrodes 1014a, 1014b indicated by the dot-and-dash lines and dotted lines, device separating areas 1017 indicated by the thin dotted lines, contacts 1006, 1007, 1009 indicated as small squares, and aluminum interconnections 1005, 1008, 1010 indicated by the solid lines.
FIG. 3A is a timing chart representing a taiming of timing the conventional color linear image sensor shown in FIG. 1.
In FIG. 3A, charges from the photodetector circuits 1001a-1001c are stored while pulses xc3x8TGa, xc3x8TGb, xc3x8TGc are in low level (storage times tTGa, tTGb, tTGc), and read into the charge transfer circuits 1003a-1003c while the pulses xc3x8TGa, xc3x8TGb, xc3x8TGc are in high level. The outputs are line outputs, respectively, representing successions of outputs from all pixels ranging from the first pixel to the last pixel in the photodetector circuits 1001a-1001c. The storage times for the photodetector circuits 1001a-1001c are equal to each other (tTGa=tTGb=tTGc).
If average voltages of all the pixels of the respective line outputs, from a reference level (no incident light applied), are represented by Vsiga, Vsigb, Vsigc, then depending on the sensitivity of the photodetector circuits 1001a-1001c, the relation of these average voltages are represented as Vsiga greater than Vsigb greater than Vsigc.
FIG. 3B is a timing chart representing a timing of driving the conventional color linear image sensor shown in FIG. 1 by changing the storage times for charges with respect to the respective colors, rather than changing the amount of exposure according to the amount of incident light.
As shown in FIG. 3B, the storage times tTGa, tTGb, tTGc for the respective colors are adjusted to yield saturated amounts of exposure SEG, SER, SEB, providing the same saturated output voltages Vsiga, Vsigb, Vsigc.
Another conventional color linear image sensor is disclosed in U.S. Pat. No. 5,105,264. The disclosed color linear image sensor is shown in FIG. 4. In the disclosed color linear image sensor, each of photodetector circuits has a shutter function.
The color linear image sensor shown in FIG. 4 comprises shutter gates 1015a-1015c, shutter drains 1016a-1016c, photodetector circuits 1001a-1001c for photoelectrically converting received light into charges, charge read circuits 1002a-1002c for reading the stored charges, charge transfer circuits 1003a-1003c for transferring the charges read by the charge read circuits 1002a-1002c, and output circuits 1004a-1004c for outputting the charges transferred by the charge transfer circuits 1003a-1003c. 
Two-phase drive clocks xc3x81, xc3x82 for driving the charge transfer circuits 1003a-1003c are supplied from pulse lines L1001a-L1001c, L1002a-L1002c that are disposed closely to the charge transfer circuits 1003a 1003c. 
The shutter gates 1015a-1015c and the shutter drains 1016a-1016c are disposed on the opposite side of the photodetector circuits 1001a-1001c, which appropriately changes the pulse width supplied to the shutter gates 1015a-1015c while pulses xc3x8TGa, xc3x8TGb, xc3x8TGc applied to the charge read circuits 1002a-1002c are in low level, enabling a production of the appropriate amounts of exposure with respect to the three colors R, G, B.
The shutter gates 1015a-1015c and shutter drains 1016a-1016c are also provided in the conventional color linear image sensor shown in FIG.4.
FIG. 5 is a timing chart representing a timing of driving the conventional color linear image sensor is shown in FIG. 4.
As shown in FIG. 5, the storage times tTGa, tTGb, tTGc for the respective colors are equal to each other (tTGa=tTGb=tTGc), so that the line outputs from the respective three colors can be outputted in synchronism with each other. Even if the photodetector circuits 1001a-1001c have different sensitivity, adjustment of the width of pulses xc3x8Sta, xc3x8STb, xc3x8STc supplied to shutter gates 1015a-1015c affords the same voltage (Vsiga=Vsigb=Vsigc) for the respective three colors.
The same voltage can be produced for the respective three colors for the following reasons: Since pulses xc3x8TGa-xc3x8TGc applied to the charge read circuits 1002a-1002c are low level While pulses xc3x8STa-xc3x8STc supplied to the shutter gates 1015a-1015c are high level, and charges stored in the photodetector circuits 1001a-1001c are drained into the shutter drains 1016a-1016c, and hence are reduced charges in the photodetector circuits 1001a-1001c to zero level. When the pulses xc3x8STa-xc3x8STc supplied to the shutter gates 1015a-1015c then become low level, an accumulation of charges starts again.
The substantial charge storage times for the signal outputs of the colors R, G, B are thus represented by tSTa, tSTb, tSTc, respectively. Based on these storage times tSTa, tSTb, tSTc, it is possible to establish appropriate amounts of exposure for the signal outputs of the colors R, G, B, and to cause the same saturated output voltage for the three colors to be outputted with an arbitrary light source.
Generally, an output voltage of a color linear image sensor is proportional to an amount of exposure, i.e., the product of an amount of incident light applied to the photodetectors and a storage time.
However, the output voltage does not increase beyond a certain output voltage level even when the amount of exposure increases. Such an output voltage level is referred to as a saturated output voltage Vsat. The amount of exposure corresponding to the saturated output voltage is referred to as a saturated amount of exposure. The greater the saturated amount of exposure is, the wider the voltage amplitude adoptable as the saturated output voltage and the dynamic range, i.e., the ratio of the saturated output and noise, e.g., dark output. Image sensors are required to have a saturated output voltage Vsat as high as possible.
The saturated output voltage is normally determined by the maximum amounts of stored charges in the photodetector circuits, the maximum amounts of charges in the charge transfer circuits, or by the amplitudes of the signal outputs from the signal output circuits. Details of the saturated output voltage will be omitted as they have no direct bearing on the present invention.
It can be seen from FIG. 8 that GREEN reaches a saturated output voltage with least amount of exposure, i.e., GREEN has the highest sensitivity, and sensitivities RED and BLUE decrease in order of RED, BLUE, and that the amount of exposure for BLUE needs to be three times the amount of exposure for GREEN.
Since color filters (not shown) are mounted respectively on three color linear image sensors in case they have the aforementioned characteristics, the saturated output voltages of the respective color linear image sensors are the same as each other for the three colors R, G, B unless the sizes of the photodetectors and the charge transfer circuits are changed or the maximum voltage amplitudes of the output circuits are changed. Because the color linear image sensors should have as high a saturated output voltage as possible, the saturated output voltages for the three colors should preferably be equal to each other.
Color linear image sensors usually have different sensitivities (output signal voltages/amounts of exposure) for R, G, B output signals. Even if sensitivities for R, G, B output signals are equal to each other under a certain light source, they may not necessarily be equal to each other under a different light source. The relationship between the amount of exposure and output voltages of color linear image sensors are as shown in FIG. 8.
As is clear from FIG. 8, whereas the saturated output voltages Vsat for the R, G, B output signals are equal to each other, RED, BLUE output, other than an GREEN output whose sensitivity is the highest, can be used only up to respective levels VsatR, VsatB. If the color linear image sensor is used beyond its saturated amount of exposure SEG, i.e., the amount of exposure which produces the saturated output voltage VsatG for the GREEN output signal, the GREEN output signal will exceed the saturated output voltage thereof, and no proper image data will be generated for GREEN. A charge overflow from the photodetectors and the charge transfer circuit for GREEN may flow into the photodetectors and the charge transfer circuit for the other two colors, leading to a color mixture.
In the example shown in FIG. 8, GREEN has the highest substantial saturated output voltage and BLUE the lowest. Therefore, a difference in the dynamic range from color to color results in an adverse effect on the quality of images generated by the color linear image sensor. It may be possible to use the color linear image sensor such that different amounts of light for the respective colors are applied to the color linear image sensor to produce saturated output voltages for the respective colors. The use of the color linear image sensor in such a manner, however, leads to a complex driving of the light source and an increase in the cost of color scanners and color copying machines.
As described above, the overflowed charges flow into the photodetector circuits for the other colors and the charge transfer circuit, so that the conventional color image sensors are unsuitable for use in intended applications.
In color scanners and color copying machines provided with a color linear image sensor, the color linear image sensor is mechanically scanned in a direction (feed direction) perpendicular to the direction (main scanning direction) of the arrays of photodetectors of the color linear image sensor.
Therefore, in order to obtain color information of all the three colors of an image in an arbitrary position on a subject, a first line, e.g., GREEN, of the image is scanned, then a second line, e.g., BLUE, of the image is scanned, and finally a third line, e.g., RED, of the image is scanned. At this time, until after the third line is scanned, color information obtained when the first and second lines are scanned needs to be stored, and then signal processing needs to be performed on the color information for the three colors. Therefore, an external memory is required to store the color information of the first and second lines. For example, Assuming that the gray scale (gradations from white to black) is represented by 12 bits, a color linear image sensor of 5,300 pixels for use in a high-resolution color scanner or color copier requires an external memory whose storage capacity is represented by:
C=5300xc3x9712xc3x973xc3x97(M+1) bitsxe2x80x83xe2x80x83(1)
where M represents the interlinear distance between two adjacent photodetector circuits, as the number of scanning cycles. The interlinear distance is produced when the absolute value of the distance between pixel columns of the color linear image sensor by the size of pixels.
For example, if R, G, B have a size of 8 xcexcmxc3x978 xcexcm and the interlinear distance between photodetector circuits is 64 xcexcm, M=64 xcexcm/8 xcexcm=8, the storage capacity of a required external memory is 1,717,200 bits.
As can be seen from the above equation (1), in order to reduce the storage capacity of an external memory, it is necessary to shorten the interlinear distances between three photodetector circuits and reduce the number of scanning cycles.
Extraordinarily large and narrow semiconductor chips for use as color linear image sensors, for example, a photodetector circuit has a length of 5300xc3x978 xcexcm=42.4 mm, and an overall length runs up to 45 to 50 mm including an output circuit and bonded regions.
Therefore, variations in the chip length increase or decrease contribute only to the formation of two or three patterns of color linear image sensors on a wafer. This scarcely affects the cost of the chip.
As can be seen from the above example of the extraordinarily long and narrow semiconductor chips, the width of the chip is the sum of the distance between photodetectors (twice the distance between lines), the width of the charge read circuits, the width of the charge transfer circuits, the width of the pulse lines, and the width of other peripheral regions, and is about 1.0 mm at most. Thus, the chip width is responsible for the chip cost. It is desirable to shorten the interlinear distance as it greatly affects the chip cost.
Major factors for determining the interlinear distance includes the size of one photodetector pixel, the size of the charge read circuits, the size of the charge transfer circuits, and the size of the device separating areas between the charge transfer circuits and the photodetector circuits adjacent thereto (including the size of the pulse lines).
In the conventional color linear image sensor shown in FIG. 2, for example, one pixel of the photodetector circuits 1001a-1001c has a size of 8 xcexcm, the charge read circuits 1002a-1002c have a size of 10 xcexcm, the charge transfer circuits 1003a-1003c have a size of 12 xcexcm, and the device separating areas 1017 between the charge transfer circuits 1003a-1003c and the photodetector circuits 1001a-1001c adjacent thereto have a size of 32 xcexcm.
With the size of 2 xcexcm of connecting areas added to the above sizes, the overall interlinear distance is 64 xcexcm (M=8).
Of the above factors, the size of one pixel of the photodetector circuits 1001a-1001c is unchangeable as it is predetermined. It is not easy to reduce the size of the charge read circuits 1002a-1002c from 10 xcexcm because the charge read circuits 1002a-1002c require interconnections for supplying a clock signal to drive them and an area for connecting the polycrystalline silicon electrodes 1014a, 1014b where the charge read circuits 1002a-1002c are fabricated. The smaller the size of the charge transfer circuits 1003a-1003c is the smaller the maximum amount of charges that can be processed and the dynamic range of output signals are in corresponding thereto. Therefore, a careless reduction in the size of the charge transfer circuits 1003a-1003c results in characteristic degradations.
The charge transfer circuits 1003a-1003c and the device separating area 1017a between the photodetector circuits adjacent thereto need the considerable length of about 30 xcexcm, As with the charge read circuits 1012a-1012c, the reduction in the device separating areas 1017 is not an easy task, because they are fabricated by connecting the aluminum interconnections 1005, 1008, 1010 which serve as the pulse lines in the device separating areas 1017 and one of the polycrystalline silicon electrodes 1014a, 1014b where the charge read circuits 1002a-1002c are fabricated with the contacts 1006, 1007, 1009, and by connecting the polycrystalline silicon electrodes 1014a, 1014b and the aluminum interconnections 1005, 1008, 1010 with the contacts 1006, 1007, 1009.
Accordingly, it is hard to change the sizes of the above areas. The interlinear distance shown in the conventional color linear image sensor shown in FIG. 2 is the least one at present.
Of the conventional arrangements described above, the conventional color linear image sensor shown in FIG. 2 does not any have shutter gates and shutter drains in the photodetectors.
In order to produce the same amount of saturated voltages in the three colors R, G, B, therefore, it is necessary to change the storage times for charges with respect to the respective times, as shown in FIG. 3B, rather than changing the amount of exposure depending on the amount of incident light. However, there have been disadvantages that the timing control for the signals is complex, and the line outputs for the lines 1, 2, 3 are out of phase with each other, so that subsequent signal processing is inevitable.
The conventional color linear image sensor disclosed in U.S. Pat. No. 5,105,264 has respective shutter gate and shutter drain at the photodetectors individually.
There has been a problem in the disclosed structure that the interlinear distance increases compared with that in the conventional color linear image sensor shown in FIG. 2. This problem becomes clear in comparing FIG. 1 with FIG.4, where the sizes of the added shutter gates and the added shutter drains become a factor in the determination of the interlinear distance.
To give an actual example, the shutter gates (including aluminum interconnections) require a size of about 10 xcexcm and the shutter drains (including aluminum interconnections) require a size of about 10 xcexcm. Therefore, the interlinear distance increases by about 20 xcexcm. If one pixel has a size of 8 xcexcm, M=84 xcexcm/8 xcexcm=10.5, resulting in an increase of 2-3 lines.
It is an object of the present invention to provide a color linear image sensor device with a shutter function, which retains substantially the same interlinear distance as that of conventional color linear image sensors without a shutter function, and which affords appropriate amounts of exposure for R, G, B.
An aspect of the present invention is to provide a method of manufacturing a color linear image sensor device with a shutter function.
According to one aspect of the present invention, there is provided a color linear image sensor device comprising first, second, and third linear image sensors having different sensitivities with respect to incident light and arranged successively in sensitivity decreasing order from the outermost, and a shutter gate and a shutter drain for adjusting an amount of exposure of the linear image sensor which has the highest sensitivity to incident light.
According to another aspect of the present invention, the color linear image sensor device further comprises means for changing the pulse interval of a first pulse applied to drain charges stored in the linear image sensor which has the highest sensitivity to incident light supplied to the shutter gate through the shutter gate to the shutter drain, thereby changing an output voltage from the linear image sensor which has the highest sensitivity to incident light.
The fabrication of the first, second, and third linear image sensors involves the steps of introducing a P-type impurity into an N-type semiconductor substrate with ion implantation and thermally diffusing the introduced P-type impurity at a high temperature to form a P-type well, introducing an N-type impurity into predetermined areas where the first, second, and third photodetector circuits are formed with ion implantation, and then thermally diffusing the introduced N-type impurity at a high temperature to form a first N-type region, introducing an N-type impurity into predetermined areas where the first, second, and third charge transfer circuits and the shutter drain are formed with ion implantation and then thermally diffusing the introduced N-type impurity at a high temperature to form second and third N-type regions, forming the shutter gate, the first, second, and third charge read circuits, and the first, second, and third charge transfer circuits in predetermined areas, using a thermally oxidized film as an insulating film, and finally introducing a P-type impurity with ion implantation with polycrystalline silicon electrodes used as masks to form a P-type region providing the first, second, and third photodetector circuits in self-alignment with the polycrystalline silicon electrodes.
According to another aspect of the present invention, there is provided a color linear image sensor device comprising first, second, and third linear image sensors for three colors having different sensitivities with respect to incident light and arranged in order of the linear image sensor whose sensitivity to incident light is the highest, the linear image sensor whose sensitivity to incident light is the lowest, and the linear image sensor whose sensitivity to incident light is middle from an outermost, a first shutter gate and a first shutter drain disposed at the linear image sensor whose sensitivity to incident light is the highest, for adjusting an amount of exposure to the linear image sensor, and a second shutter gate and a second shutter drain disposed at the linear image sensor whose sensitivity to incident light is middle, for adjusting an amount of exposure to the linear image sensor.
According to an aspect of the present invention, the linear image sensor whose sensitivity to incident light is the highest comprises a first photodetector circuit for converting the incident light into charges, a first charge read circuit for reading the charges generated by the first photodetector circuit, a first charge transfer circuit for transferring the charges read by the first charge read circuit in synchronism with first and second drive clocks, a first output circuit for converting the charges transferred by the first charge transfer circuit into a voltage, and outputting the voltage, and first and second pulse lines for supplying the first and second drive clocks to the first charge transfer circuit.
The linear image sensor whose sensitivity to incident light is the lowest and the linear image sensor whose sensitivity to incident light is the second highest comprise respective second and third photodetector circuits for converting the incident light into charges, respective second and third charge read circuits for reading the charges generated by the second and third photodetector circuits, a second charge transfer circuit for transferring the charges read by the second and third charge read circuits in synchronism with second and third drive clocks, a second output circuit for converting the charges transferred by the second charge transfer circuit into a voltage, and outputting the voltage, and a third pulse line for supplying the third drive clock to the second charge transfer circuit.
The second charge transfer circuit, the signal output circuit, and the first and third pulse lines are shared by the linear image sensor whose sensitivity to incident light is the lowest and the linear image sensor whose sensitivity to incident light is middle.
According to an aspect of the present invention, the above color linear image sensor device further comprises means for changing a pulse interval of a first pulse applied to drain charges stored in the linear image sensor which has the highest sensitivity to incident light and supplied to the first shutter gate, through the first shutter gate to the first shutter drain, and a pulse interval of a second pulse applied to drain charges stored in the linear image sensor which has middle sensitivity to incident light and supplied to the second shutter gate, through the second shutter gate to the second shutter drain, thereby changing output voltages from the first and third linear image sensors.
The fabrication of the first, second, and third linear image sensors of the above color linear image sensor device involves the steps of introducing a P-type impurity into an N-type semiconductor substrate with ion implantation and thermally diffusing the introduced P-type impurity at a high temperature to form a P-type well, introducing an N-type impurity into predetermined areas where the first, second, and third photodetector circuits are to be formed with ion implantation, and then thermally diffusing the introduced N-type impurity at a high temperature to form an N-type region, introducing an N-type impurity into predetermined areas where the first, second, and third charge transfer circuits and the shutter drains are to be formed, with ion implantation, and then thermally diffusing the introduced N-type impurity at a high temperature to form N-type regions, forming the shutter gates, the first, second, and third charge read circuits, and the charge transfer circuits in predetermined areas, using a thermally oxidized film as an insulating film, and introducing a P-type impurity with ion implantation with polycrystalline silicon electrodes used as masks to form a P-type region providing the first, second, and third photodetector circuits in self-alignment with the polycrystalline silicon electrodes.
The color linear image sensor devices according to the present invention have the same interlinear distance as that of color linear image sensors with no shutter function, and allow the amount of exposure to be adjusted for the linear image sensor whose sensitivity to incident light is the highest.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the resent invention.